docstr: check zero crossing rate on a buffer with a negative impulse 
docstr_tok: ["check","zero","crossing","rate","on","a","buffer","with","a","negative","impulse"]
code: def test_negative_impulse(self):
      """check zero crossing rate on a buffer with a negative impulse"""
      self.vector[int(buf_size / 2)] = -1.0
      self.assertEqual(2.0 / buf_size, zero_crossing_rate(self.vector))
  
file_path: aubio/python/tests/test_zero_crossing_rate.py
imports: from numpy.testing import TestCase
  from aubio import fvec, zero_crossing_rate
label_desc: Use image node_id 5 for calling the zero_crossing_rate_test_case obj's underlying member method code with example usage: obj.test_negative_impulse() without return types